(1) Field of the Art
The invention relates to a method of producing a titanium hot coil, especially to a method of obtaining a titanium hot coil having excellent surface quality by continuously producing a high quality titanium hot rolled strip of a varying thickness using a continuous hot strip mill and tightly taking up the resulting hot rolled strip in a coil form without causing those problems such as large telescopic coiling, friction dig, etc. which would otherwise occur frequently during the winding operation of the rolled strip immediately after rolling.
(2) Description of the Prior Art
A titanium material has excellent chemical and mechanical properties such as good corrosion resistance, heat resistance and abrasion resistance, and high specific strength. Owing to these outstanding properties, the titanium material has gained in recent years a wide range of application as an excellent material for airplanes, heat exchangers, apparatuses for converting brine into fresh water, electric power plants, apparatuses for the chemical industry and so forth. The demand for this material will be further increasing in the future.
At present, however, the production of the titanium strip has been carried out on a limited scale mainly using a Steckel mill. The Steckel mill consists of two sets of coilers and a 4-High reversible rolling mill interposed between the coilers whereby the titanium slab is passed through the 4-High reversible rolling mill, alternately taken up by the two coilers and caused to reciprocate in a required number of reciprocation so as to gradually reduce its thickness and thus to obtain a titanium strip having a desired thickness.
However, this method involves the problems that not only the mass-production is infeasible, but also the dimensional accuracy of thickness is low. In addition, the method is not free from the problems such as inferior shape of the camber and frequent occurrence of surface defects due to the scale.
It is therefore an earnest desire for those concerned in the art to develop a novel mass-production system which would replace the abovementioned Steckel mill system and enable to produce high quality titanium strip on a large scale at a low production cost in high yield. Most desirable and advantageous mass-production system would be one which uses a continuous hot rolling system and to which a hot strip mill for steel is adaptable.
Unlike the steel, however, titanium is extremely reactive, has a small specific gravity and its stress-strain characteristics is extremely sensitive to a temperature change. Because of these properties, the continuous hot rolling process of the titanium strip over its entire production steps ranging from heating, rolling and winding involves wide and difficult technical problems that are remarkably different from those encountered in rolling of the steel as will be described elsewhere in this specification. For this reason, the industrial production of the titanium strip has not yet been established in accordance with the continuous hot rolling system.
In comparison with the steel, the production of the titanium strip by the hot rolling is more difficult. Especially, large telescopic coiling tends to occur during the take-up operation of the strip after rolling, and it is not easy to obtain a normal coil tightly wound in an orderly manner. This is a phenomenon peculiar to the winding of the titanium strip that cannot be observed in the winding of the steel strip.
Occurrence of this large telescopic coiling not only constitutes a serious obstacle in performing a series of the entire production steps of the continuous hot rolling, but also means the production of an inferior product as such. Even if the degree of large telescopic coiling is not much serious, it induces mutual contact of the surface of strip during winding whereby so-called "friction dig" (8) of a recessed form take place over the wide range on the surface of the strip (6) and extremely deteriorate the surface quality of the coil. At times, a produced coil becomes defective as a whole lot.
Incidentally, as the prior art most relevant to the present invention, mention can be made of U.S. Pat. Nos. 3,169,085, 3,496,755, 3,492,172 and 3,481,799.